Make a Better Rain Garden

My model is good enough for the National Botanical Garden

By Dr. Francis Gouin

The purpose of rain gardens is to reduce surface runoff by capturing water in ponds where it can infiltrate the soil. Many rain gardens begin with dug ponds lined with sand and gravel. Water-tolerant plants added in and around the ponds absorb more water.
This design can absorb only a limited amount of water based on the soil porosity, a measure of texture and compaction. After a heavy rain, water can stand for days and weeks, so the gardens become breeding grounds for mosquitoes. Even tolerant plant species have problems surviving standing water.
There’s also a septic smell to some newly constructed rain gardens. The odor is caused by soils containing more than three percent organic matter, which is typical with a manufactured medium with compost blended in. When soils and materials rich in organic matter are water-logged, they undergo anaerobic digestion, resulting in odor.

Getting It Right A well-designed and constructed rain garden should not retain water for more than a couple of days and should promote the growth of plants tolerant to wet soil conditions. Soil for its bottom should contain only well decomposed organic matter, not freshly made compost.
Here’s how to meet both those goals.
Water absorbing capacity can be significantly increased by either auguring holes in the bottom of the rain garden during construction or by trenching.
If the rain garden is big enough to accommodate a power trencher, trenches four feet wide and up to four feet deep should be dug at 18- to 24-inch intervals across the bottom of the pond.
Fill trenches with pine fines in 12-inch lifts. Pack the pine fines using a eight-foot four-by-four timber between each lift until the trenches are filled. Finally, place a covering of sand or gravel over the bottom of the pond.
In small ponds, augur four- to 10-inch diameter holes spaced about 18 inches apart to a depth of three to four feet. Fill the holes with pine fines in 12-inch lifts and packed similarly. Cover the bottom as above.
Pine fines are the fine particles that collect in the manufacture of pine bark mulch. They contain 100 percent lignins, which resist decomposition. When buried deep in the soil and covered with sand or gravel, they will not generate odors. Pine fines are also a rich source of humic and fulvic acids. Both of these naturally occurring acids will help loosen the soil, allowing it to absorb more water. Further, the pine fines will serve as a wick, pulling water down where it can be better absorbed.
Augering or trenching deep into the sub-soil greatly increases its absorbing capacity. This system also increases the surface area and water-absorbing capacity of the soil.
To accommodate plants in the pond, place a four- to six-inch layer of a sandy loam soil with two to three percent natural organic matter over the layer of sand or gravel. Never amend the soil with perlite or vermiculite. Perlite will deteriorate into slime after several years of freezing and thawing. Vermiculite flattens into plate-like particles in only six to eight months after they have absorbed water.
I have used this system many times and never had a failure. The largest project I was involved in was the National Botanic Garden at the base of Capital Hill in Washington, D.C. Following heavy rains, the existing water gardens overflowed into the gardens. To increase the water garden’s ability to absorb more water, we augered 10-inch holes in the bottom to a depth of five feet at 24-inch intervals. The holes were packed and the bottom covered with gravel as described.

Apologies to Flint, Michigan

In my June 29 column, The Poop on Biosolids, I wrote “Unless the biosolids come from Flint, Michigan, the lead levels in Class A biosolids are far below EPA standards in Compro, Orgro and Earthlife. The same is true for cadmium.”
A Bay Weekly online reader in Flint who is knowledgeable about the biosolids has corrected me. He has assured me that Flint is generating Class A biosolids. The assumption that I made was based on the research I did with biosolids from Baltimore in the late 1970s before Mayor Schaffer cleaned up the sewer system. I apologize for making that assumption.

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